The invention relates to a device and a method for exposing a digital image onto light-sensitive material, which includes a computer unit for splitting the digital image into n partial digital images, where nxe2x89xa72, at least one control unit for controlling n reflective light modulators, pixel by pixel, for generating the n partial images, an illumination unit for illuminating the light modulators and an objective for reproducing the partial images onto the light sensitive material. Such devices and methods are known from various references dealing with digital exposure devices.
The German Patent No. 42 11 128 discloses one example that describes an image transfer method. With this method, an image original is scanned point by point and line by line, the individual signals are digitized and the digital image signals are reproduced on photographic paper via an optical system using a light modulator that is illuminated by a light source.
In particular, with such exposure devices it has also been known to split the digital images into partial images. Each partial image is then displayed on a light modulator. Using an optical system, the partial images are reproduced next to one another on the light-sensitive material resulting in the total image being presented on this material. Because the resolution of the light modulator must always be sufficient only for a part of the image, even larger images can be exposed in this manner with a sufficient resolution without the need to use expensive extreme high-resolution image generation devices. However, with this procedure it is always problematic to reproduce those areas where the partial images connect such that no density jump occurs in the image.
One example for such an exposure method is described in the U.S. Pat. No. 5,801,814. Here, the original digital image is split into numerous partial images. One after the other, the partial images are generated using an LCD and exposed onto the light-sensitive material one next to the other using a projection lens system. Placing the partial images of the LCD image next to one another to combine them to a total image on the light-sensitive material is accomplished such that either the LCD, the objective, a beam splitter inserted into the beam path or the light-sensitive material is moved between the exposures of the partial images. To avoid density jumps in the connecting zones of consecutive partial images, the partial images are exposed overlapping by a few pixels. In the overlapping area, the density of the pixels of each partial image is then reduced inversely proportional to the number of overlapping partial images. For a precise recalibration of varying densities transmitted by individual pixels, sensors are inserted into the beam path of the LCD""s overlapping areas prior to exposure of the images, such that the pixels with multiple exposure can be measured and corrected.
This method is suitable for exposure processes where a fast production of many images is not required. For example, it lends itself to the generation of infrequent index prints between the standard exposures. In this case, the time loss resulting from the consecutive exposure of partial images and also the calibration required between exposures can be tolerated. However, with this method, the exposure of an entire digital image takes too much time to be used in a photographic reproduction device where images are to be exposed continuously.
Another system, which is particularly suitable for a line exposure device for consecutive exposure of two partial images is described in the, U.S. Pat. No. 5,105,299. Here, two lines arranged underneath one another of a DMD (so-called Digital Micromirror Device or Deformable Mirror Device) are illuminated by a lamp and the light is modulated by the small mirrors of the DMD according to the image data such that one half of the image line is generated on each DMD line. The modulated and reflected light of the two image line halves is redirected using an optical system consisting of reflective prism surfaces, such that lines that are located underneath one another in the object, each representing one image half of a line, are displayed next to one another in the image. This results in a consecutive line on the material to be exposed.
In the same manner as with all other digital exposure devices, where on paper partial images are combined to a total image, the problem of a density jump at the connecting edges of the partial images occurs, of course, here as well. However, the patent document does not deal with this problem.
A particular problem of this arrangement stems from the fact that the illumination and reproduction beam paths must be spatially separated sufficiently such that the reproduction beam path is not vignetted by the illumination optics. This spatial separation prevents a compact design of the exposure system.
This problem is addressed by the device disclosed in the U.S. patent application Ser. No. 09/495,802. This device combines the light reflected by two DMDs by means of a beam splitter. The DMDs are arranged on both sides of the beam splitter such that the light of one DMD passes through one half of the beam splitter. The light of the other DMD is reflected by the second half such that images generated by the DMDs are reproduced next to one another on the light-sensitive material. For the illumination, a light source that illuminates the DMDs passing by the beam splitters and the reproducing objective is provided for each DMD. To correct the connecting area of the consecutively reproduced partial images, this application recommends to partially decouple the combined partial images from the reproduction beam path using a beam splitter. The decoupled light is guided to a sensor where the exposure in the connecting zone is monitored for uniformness. In this manner, a continuous correction of the density in the connecting zone can be carried out even during the exposure. Similar to the previous arrangement, the disadvantage with this one is also that a very compact design is impossible because the illumination and reproduction beam paths must be spatially separated. Furthermore, it is problematic with such designs to adjust the two light modulators such that their images connect precisely on the photographic paper.
It is, therefore, the principal objective of the present invention to develop a device for exposing digital images, where partial images generated at various light modulators are reproduced simultaneously next to one another on light-sensitive material, such that, with a compact, adjustment-friendly design of the device, no density jumps occur at the connection areas of the partial images.
This objective, as well as other objectives which will become apparent from the discussion that follows, are achieved according to the invention by a device and a method wherein the generated partial images are combined by a prism or a beam splitter complex, arranged between the illumination unit, light modulators and the objective, such that both the illumination and the reproduction beam path pass through it.
A very compact design of the exposure device can be accomplished by the arrangement of beam splitters or beam splitter prisms between the illuminating unit, light modulator and reproduction objective according to the invention, because both the illumination and the reproduction beam paths can be directed through the beam splitters or prisms. By directing the beams in this manner, it is possible to attach the reflecting light modulators directly to the complex that combines the partial images. On one hand, this makes possible the extremely compact design of the exposure device, and on the other hand, it becomes comparatively easy to adjust the light modulators due to the attachment of the light modulators on the complex. For example, using a glass adhesive they can be placed on the complex in a movable fashion and then moved around until an uninterrupted total image is achieved when reproducing a test image on the light-sensitive material. Thereafter, the glass adhesive is hardened. Once adjusted, the light modulators remain in a fixed position on the complex; even the replacement of components of the exposure device, for example, of the illumination unit or the objective, would not require a new adjustment.
When using prism complexes, these are designed and arranged such that beams emitted by the illumination device strike the prism interface at such a small angle that they are completely reflected onto the light modulator at the respective interface at the transition to the optically thinner medium. The beams reflected by the light modulator strike the interface of the beam splitter prisms at a different angle than the incident beams such that they can pass the interfaces. During the transmission, the beams are deflected such that behind the beam splitter prisms together with beams reflected by other light modulators they result in a common beam bundle, which exposes an uninterrupted image onto the light-sensitive material.
DMDs (Digital Micromirror Devices), VFDs (Vacuum Fluorescence Displays), reflective LCDs, GLVs (Grating Light Valves) or any other reflective light modulators can be used as the light modulators.
Depending on the design and arrangement of the beam splitter complex and the illumination design, partial images of any desired number of light modulators can be combined. Although an even number of light modulators is easier to implement for symmetry reasons, basically an uneven number of light modulators can be combined as well.
Both physical and geometrical beam splitters are conceivable for beam splitting. Metallic physical beam splitters have the advantage that regarding the color, they barely alter the split beam, such that the combined image does not require color correction; however, their disadvantage is a high light loss.
All potential multi-color light sources, such as halogen lamps or LEDs, whose light can be combined in a common beam path using a beam splitter are suitable as illumination units. For the illumination, it is only important that the light for all light modulators originates from the same light source, if at all possible. Depending on the design of the beam splitter complex, the light of the illumination unit can be directly directed to the light modulators, or the light of the illumination unit is initially split by additional beam splitters such that several virtual images of this one light source are generated that each illuminate one light modulator. The advantage of having only one illumination unit for illuminating several light modulators is that changes of the illumination unit affect all partial images generated by the light modulators in the same manner, such that no density jumps due to differing fluctuations at the connecting edges of the partial image are recognizable in the total image.
The design of illumination unit, illumination optics, beam splitter complex, light modulators as well as reproduction optics should be symmetrical such that both the beam paths that illuminate the light modulators and the light paths that reproduce the image generated at the light modulators onto the light-sensitive material have the same length for each light modulator. This is the prerequisite for a Kxc3x6hler beam path to be realized. For this, an illumination optics must be designed and arranged such that the image of the light source is reproduced in the pupil of the reproduction optics and the reproduction optics must be designed such that the pupil of the illumination optics at the same time becomes the port of the reproduction optics. Such a beam path ensures that no location information of the lamp is transferred into the image. Eliminating any location information and using only one light source prevents time-related changes that occur locally at one place in the lamp from being visible at the edge of a partial image but not at the edge of the connecting one, which would result in density jumps at the connecting area.
In place of the implementation of a Kxc3x6hler beam path, the beam path for preventing density jumps can also be designed such that the light reproducing the connecting area of the partial images stems from the same place of the illumination unit.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.